I have found that a rice cake will have great shredding appeal when added to toys. My favorite application is to top an empty clear plastic applesauce cup that has nuts, small pieces of vegetable and fruit inside with a rice cake. My parrots will eagerly shred through the rice cake to get at what is inside the applesauce cup.
I find my birds are more interested in shredding away the rice cake to get at what is under it - than they are in eating the rice cake. But it is good to know that there are whole grain rice cakes available that are made with brown rice and no added salts or sugars.
Recently, in a quest to give my parrots something new and exciting to do; I discovered ice cream cones make great additions to toys. They have the same shredding appeal as the rice cakes and they add variety. I will make a small hole in the end of the ice cream cone and string it on the toy (upside down). Inside the ice cream cone I will put nuts and other treats and then cover the cone with an empty applesauce cup. Above and below those items I add other toys like a crunch ball, whiffle ball, corn husks, plastic toy parts, etc.
In the example pictured below, I used a vine ring as my base and medium hemp rope to string on items to make the vine ring more interesting. I placed a rice cake on top. In the center of the vine ring I filled an ice cream cone with a nutri-berry and carrot pieces and topped it with a vine ball stuffed with an almond. Under the vine ring I added a paper towel roll stuffed with corn husks and another vine ball with a peanut inside. After Zorba tore apart the ice cream cone and rice cake, he went to work shredding the vine balls and vine ring.